1. Field of the Invention
The present invention relates to a constant current generator that minimizes silicon resources and noise contribution.
2. Discussion of the Related Art
In many integrated circuits, the analog sections can be biased using either constant currents or constant “IR” currents. A constant current Ic can be generated by applying a fraction (f) of a bandgap reference voltage (VREF) across an external resistor (Rext), i.e. Ic=f*VREF/Rext. FIG. 1A illustrates an exemplary voltage-to-current converter 100 for generating a constant current Ic. In converter 100, a bandgap reference voltage circuit 101 can generate a reference voltage VREF, which is provided to a positive terminal of an operational amplifier 102. The output terminal of operational amplifier 102 is connected to a gate of an NMOS transistor 103. The source of NMOS transistor 103 is connected to a low voltage source VSS via an external resistor 104 as well as to the negative terminal of operational amplifier 102. The drain of NMOS transistor 103 provides the constant current Ic.
In a typical embodiment, bandgap reference voltage circuit 101 includes another operational amplifier. For example, FIG. 1B illustrates an exemplary bandgap reference voltage circuit 101 that includes an operational amplifier 116, which provides its output to a node 117. The collectors and bases of two pnp transistors 113 and 115 are connected to low voltage source VSS. The emitter of pnp transistor 113 is connected to node 117 via two resistors 112 and 111, which are connected in series. A node between resistors 111 and 112 is connected to the negative input terminal of operational amplifier 116. The emitter of pnp transistor 115 is connected to the positive input terminal of operational amplifier 116 and to a resistor 114, which is also connected to node 117.
A bandgap reference voltage VREF can be created by adding a diode voltage, which has a well-known negative temperature coefficient, with a voltage that is proportional to absolute temperature (ptat) in such a way that the temperature coefficient of the combination is nearly zero. In the configuration of bandgap reference voltage circuit 101, resistors 111, 112, and 114 as well as pnp transistors 113 and 115 can be appropriately sized to ensure that the temperature coefficient of VREF is balanced, i.e. substantially zero. This balancing can occur when VREF is approximately 1.22 V.
A constant IR current can be similarly generated by applying a fraction of a bandgap reference voltage across an internal resistor (e.g. polysilicon) (Rint), i.e. constant IR current=f*Vbg/Rint. Because a device may require both a constant current as well as a constant IR current, such a device generally includes a bandgap reference voltage circuit (e.g. bandgap reference voltage circuit 101) and two voltage-to-current converters, i.e. a first voltage-to-current converter for the constant current (e.g. converter 100) and a second voltage-to-current converter for the constant IR current (identical to converter 100, wherein resistor 104 is an internal resistor).
Unfortunately, the bandgap reference voltage circuit and its corresponding voltage-to-current converters have many components that use significant area on an integrated circuit. Moreover, distributing a reference voltage can undesirably contribute to noise and offset in a device. An ideal voltage reference presents a zero impedance source of voltage. Unfortunately, interconnections on integrated circuits have finite impedances, which may allow noise from adjacent traces to be capacitively coupled to a voltage reference line. Further, a voltage reference must be compared to some other voltage, typically “ground.” If the absolute voltage of “ground” at the point of voltage reference generation is not the same as “ground” at the point where the reference is used, then the reference voltage will appear to have an error equal to the difference in ground potentials. A difference in ground potentials is a common problem in large-scale integrated circuit design. Additionally, as process technologies scale and voltage supplies are lowered, even generating bandgap reference voltages becomes challenging. For example, positive voltage supplies can now be at 1.8 V or below.
Therefore, a need arises for a more compact, noise minimizing constant current generator that can operate with a positive voltage supply equal to or less than 1.22 V.